A bipolar junction transistor (BJT) is a type of transistor that relies on the contact of two types of semiconductor material for its operation and can be found either as individual discrete components, or in large numbers as parts of integrated circuits. A BJT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region creating two pn-junctions, known as the emitter-base junction and the collector-base junction. Typically these regions are created by doping a semiconductor material, such as silicon, with different dopants to create a p-type and an n-type material.
Bipolar transistors come in two types, or polarities, known as PNP and NPN based on the doping types of the three main terminal regions. The PNP type BJT uses an n-type material for the base region and p-type material for the emitter region and collector region. The NPN type BJT uses a p-type material for the base region and n-type material for the base region and collector region. The following discussion is generally in the terms of NPN transistors, but it is to be understood that the discussion is applicable to PNP transistors as well.
The heterojunction bipolar transistor (HBT) is a type of BJT that uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. For example, an emitter and collector may use doped silicon while the collector uses a silicon-germanium alloy.
Bipolar transistors, for example, as used in bipolar complementary metal-oxide semiconductor (BiCMOS) processes, include an emitter formed as a pedestal over a planar base region that overlies a collector. The emitter is bounded on opposing sides by dielectric spacers, which overlay a portion of the base region. The emitter is typically formed by depositing a layer of doped polysilicon over the base region and dielectric spacers, and defining the emitter using standard photolithographic patterning.
The base region directly under the junction with the emitter is typically referred to as an intrinsic base region, while the base region falling outside of the emitter and the spacer is referred to as an extrinsic base region. Alignment of the various steps used to fabricate the various regions of the base region, the base-emitter junction, and the emitter can be difficult to control.